1. Field of the Invention
The present invention relates to a liquid crystal alignment film. More particularly, it relates to a novel liquid crystal alignment film which can readily be prepared on an industrial scale and whereby a large tilt angle of liquid crystal molecules and stabilized alignment characteristics can be obtained.
2. Description of the Prior Art.
Liquid display devices are display devices which utilize electrooptical changes of liquid crystals, and they are small in size and light in weight and have a feature that their power consumption is small. Accordingly, they have found remarkable developments in recent years as display devices for various displays. Among them, a twisted nematic type (TN type) electric field effect liquid crystal display device is a typical example wherein a nematic liquid crystal having a positive dielectric anisotropy is used, so that liquid crystal molecules are aligned in parallel with a pair of mutually opposing electrode substrates at the respective interfaces, and the two substrates are arranged so that the alignment directions of the respective liquid crystal molecules will cross each other.
In such a TN type liquid crystal display device, it is important to align liquid crystal molecules so that their axial directions will be uniformly in parallel with the surface of a substrate and so that the liquid crystal molecules will have a certain inclination angle (hereinafter referred to as a tilt angle) to the substrate. The following two methods are known as typical methods for aligning liquid crystal molecules in such a manner.
The first method is a method in which an inorganic substance such as silicon oxide is vapor-deposited obliquely to the substrate to form an inorganic film on the substrate, so that liquid crystal molecules are aligned in the direction of vapor deposition. This method is not efficient from the industrial point of view although it is thereby possible to attain stabilized alignment with a certain specific tilt angle.
The second method is a method wherein an organic coating film is formed on the surface of a substrate, and its surface is rubbed in a certain predetermined direction with a cloth of e.g. nylon or polyester, so that liquid crystal molecules are aligned in the direction of rubbing. By this method, it is possible to attain stabilized alignment relatively easily. Accordingly, this method is principally employed for industrial purpose. As the organic film, polyvinyl alcohol, polyoxyethylene, polyamide or polyimide may, for example, be used. However, the polyimide is most commonly used in view of the chemical stability, thermal stability, etc. However, the tilt angle obtainable by rubbing the polyimide is usually at a level of from 1.degree.to 3.degree., and it has been difficult to attain a larger tilt angle.
In the field of liquid crystal alignment films, it has been difficult to obtain a large tilt angle constantly by a method of rubbing an organic film of polyimide or the like. As a means to solve such a difficulty, Japanese Unexamined Patent Publication No. 297819/1987 proposes a treating agent for liquid crystal alignment which is composed of a reaction product of a long chain alkylamine with a polyimide precursor. Further, Japanese Unexamined Patent Publications No. 262527/1989 and No. 262528/1989 propose an agent for liquid crystal alignment which is composed of a mixture comprising a long chain alkyl compound and a polyimide precursor. Still further, Japanese Unexamined Patent Publication No. 25126/1989 proposes a treating agent for liquid crystal alignment which is composed of a polyimide prepared from a diamine having an alkyl group. Thus, many attempts have been made to increase the tilt angle of liquid crystal by introducing an alkyl group into a polyimide, and it has been possible to increase the tilt angle to some extent. On the other hand, however, there has been a new problem such that when an alkyl group is introduced into a polyimide, the polyimide surface tends to be water-repellent, whereby wettability of liquid crystal to the polyimide surface tends to be low. If the alkyl group is excessively introduced into a polyimide to increase the tilt angle, the wettability of liquid crystal tends to be low, and in an extreme case, failure in the liquid crystal alignment is likely to result. Consequently, the display performance of the liquid crystal display device tends to be poor.
Accordingly, it has been desired to develop a polyimide for alignment films whereby a large tilt angle can be obtained by rubbing and which provides adequate wettability with liquid crystal and excellent properties for alignment.
Depending upon the aligned states of their molecules, liquid crystals are classified into nematic, smectic and cholesteric. However, a common feature is that long axes of constituting molecules are aligned in parallel with one another. As prerequisites for a certain compound to exhibit a liquid crystal property from such a regular molecular alignment, there may be mentioned (1) that the molecular structure has a geometric shape suitable for the parallel alignment, and (2) that an intermolecular attraction sufficient to maintain the parallel alignment, is maintained. Because of these two prerequisites, a compound to exhibit a liquid crystal property is required to have a rod-like or a plate-like molecular shape, and further have a permanent dipole or a readily polarizable chemical bond group (functional group). Further, the type of a liquid crystal phase is said to be determined by a proper balance of the intermolecular attraction as between the terminals and between the side surfaces of such a rod-shaped molecule ("Most Recent Technology for Liquid Crystal" edited by Shoichi Matsumoto and Ichiyoshi Kakuta, p. 62, 1983, published by Kogyo Chosakai).